DNA polymerases are useful enzymes for reagents for genetic engineering, and the DNA polymerases are widely used for nucleotide sequencing of DNA, DNA labeling, site-directed mutagenesis, and the like. Also, thermostable DNA polymerases have recently been remarked with the development of the polymerase chain reaction (PCR) method, and various DNA polymerases suitable for the PCR method have been developed and commercialized.
Presently known DNA polymerases can be roughly classified into four families according to amino acid sequence homologies, among which family A (pol I type enzymes) and family B (.alpha. type enzymes) account for the great majority. Although DNA polymerases belonging to each family generally possess mutually similar biochemical properties, detailed comparison reveals that individual enzymes differ from each other in terms of substrate specificity, incorporation efficiency of a substrate analog, primer extensibility and extension rate, mode of DNA synthesis, association of exonuclease activity, optimum reaction conditions of temperature, pH and the like, and sensitivity to inhibitors. Therefore, those possessing most appropriate properties for the applications have been selected among all available DNA polymerases, and the selected DNA polymerase has been used.
A hyperthermophilic archaebacterium Pyrococcus furiosus has produced a DNA polymerase belonging to .alpha. type, and its gene has already been isolated [Nucleic Acids Research, 21, 259-265 (1993)].
As DNA polymerases, in addition to ones expressing their functions with only one kind of an enzyme protein, such as the pol I type enzyme or the .alpha. type enzyme, there have been known oligomer enzymes constituted by a large number of subunit proteins. In addition to the protein serving as a DNA polymerase, there have also been known some cases where protein molecules for regulating their functions coexist.